The extracellular matrix of the alveolar structures is a critical determinant of lung structure and function in health and disease. Tissue culture studies have demonstrated a variety of lung cells both synthesize and destroy extracellular matrix complements. The fibroblasts synthesize collagen types I and III as well as producing a collagenase that destroys both collagen types. It also synthesized fibronectin, a glycoprotein important in cell-cell and cell-matrix interactions. The rate of collagen-synthesis is rigidly controlled. However, there is environmental stimuli such as prostagladins and mediators of activated lymphocytes can alter the differentiated state of fibroblasts with respect to collagen synthesis. The intracellular degradation of collagen has been defined: approximately 1/3 of all collagen synthesis by fibroblasts is destroyed within the cell prior to secretion. The portion of intracellular degradation can be modulated by altering the structure of collagen or by changing the levels of intracellular cyclic AMP.